1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a sheet.
2. Description of the Related Art
Recently, smaller size and higher speed have been demanded in an image forming apparatus. Such image forming apparatus is often accompanied by technical problems. That is, for example, heat is transferred to a recording material in the fixing means, so that the conveyed recording material becomes a heat source and the temperature of the entire apparatus is raised.
As a second problem, in the recording material receiving heat, when sheets are discharged and stacked continuously, sheets are adhered to each other between the face and back side of the recording material. Sticking of sheets is likely to occur on OHT (Over Head Transparency) sheets or thick paper when the heating performance is improved in the fixing device or when sheets of thin paper printed on both sides are stacked up continuously, with an attempt to improve the fixing performance of images.
An important object is how to cool the recording material effectively after fixing.
It has been generally proposed to install a cooling fan for blowing air in the conveying path after fixing and cool off the heat applied to the recording material. Japanese Patent Application Laid-Open (JP-A) No. 2002-082591 discloses an image forming apparatus including size detecting means which detects the size of recording material, and an air duct for blowing fresh air into the recording material after fixing by blowing means. In this image forming apparatus, the wind width of the fresh air blown to the recording material is adjusted in a direction orthogonal to the conveying direction of the recording material based on the detection signal by the size detecting means.
Japanese Patent Application Laid-Open No. 2004-109732 proposes to realize a further cooling effect of the recording material by disposing a cooling roller at the downstream side of the conveying direction from the fixing device, and cooling the cooling roller by blowing air to the cooling roller with a cooling fan. A higher speed has been demanded not only in recording material with maximum width, but also in recording material with small width.
When recording materials with smaller width than the maximum width are fixed continuously by the fixing device, the temperature of non-passing region not contacting with the recording materials in the heating roller of the fixing device is raised excessively. When recording materials of small width pass the fixing device continuously, heat is locally accumulated in the non-passing region not contacting with the recording materials by the portion not deprived of heat by the passing recording materials. This phenomenon is called temperature rise in non-passing region or temperature rise in end region of the fixing device. When the non-passing portion of the fixing device is raised in temperature, the components of the fixing device and the pressure roller opposite to the heating roller exceed the temperature rise limits, and these components are lowered in precision. This phenomenon of temperature rise in non-passing portion or temperature rise in end portion is particularly obvious in the heating roller of thin wall and small heat capacity.
To prevent temperature rise in non-passing portion, a structure of installing a blowing fan for blowing air to the non-passing portion of the roller pair of the fixing device and suppressing temperature rise in the non-passing portion is proposed in Japanese Patent Application Laid-Open No. 04-051179.
It may be considered to provide the fixing device with both a cooling fan for cooling the recording materials and a fan for non-passing portion for cooling the non-passing portion of the heating roller. In this structure, the air from the cooling fan for cooling the recording materials and the air from the non-passing portion fan for cooling the heating roller are converged and exhausted out of the device, but if the flow rate is excessive, exhaust may not be sufficient. The portion of air not exhausted out of the device returns to the conveying path before fixing by way of the outer circumference of the fixing device.
The air after cooling of recording materials or the air after cooling of non-passing portion of the heating roller is high in temperature. When such air returns to the conveying path before fixing, temperature rises in the transfer means for transferring unfixed image on the recording material or the image forming portion, and it has adverse effects on the image forming portion. For example, the toner may be melted in the image forming portion due to the temperature rise in the image forming portion. Further, as the device is reduced in size and the conveying speed of recording materials is increased, the flow rate of the cooling fan must be increased, and it is feared that adverse effects may be increased on the image forming portion due to return of air into the conveying path before fixing. If the air stream returning to the upstream side of the fixing device is not caused by the air after cooling the non-passing portion of the heating roller or recording materials, when the air after cooling the non-passing portion of the heating roller or recording materials remains in the device, a temperature rise in the device occurs, and other problems may be induced.